1. Field of the Invention
The present invention relates to a spark plug used in an internal combustion engine.
2. Description of the Related Art
Conventionally, a spark plug for an internal combustion engine such as an automobile engine employs a Pt (platinum) alloy chip welded to an end of an electrode for use as a spark discharge portion with improved spark consumption resistance. However, due to expensiveness and a relatively low melting point of 1769.degree. C., platinum is not satisfactory as a spark consumption resistant material for spark plug use. Thus, use of Ir (iridium), which is inexpensive and has a higher melting point of 2454.degree. C., as a material for a chip has been proposed. However, since Ir tends to produce a volatile oxide and be consumed at a high temperature zone ranging from 900.degree. C. to 1000.degree. C., a spark discharge portion formed from Ir involves a problem of consumption stemming from oxidation/volatilization rather than spark consumption. Accordingly, an Ir chip shows good endurance under low temperature conditions as in traveling in an urban area, but has a problem of a significant reduction in endurance in continuous high-speed traveling.
Thus, an attempt has been made to suppress consumption of a chip stemming from oxidation/volatilization of Ir by adding an appropriate element to an alloy used as a material for a chip. For example, Japanese Patent Application Laid-Open (kokai) No. 9-7733 discloses a spark plug whose chip is improved in high-temperature heat resistance and consumption resistance by suppressing oxidation/volatilization of Ir through addition of Rh (rhodium).
However, an Ir--Rh alloy used as a chip material in the above-disclosed spark plug must contain a considerably large amount of Rh against consumption stemming from oxidation/volatilization in a continuous high-speed, high-load operation of an internal combustion engine. Since Rh is several times more expensive than Ir and has a relatively low melting point of 1970.degree. C. as compared with that of Ir, an excessively large Rh content not only pushes up material cost of a chip but also involves insufficient resistance to spark consumption. That is, in recent years, operating conditions of spark plugs tend to become severer in association with an improvement in performance of internal combustion engines. Therefore, when such a chip is made from an Ir--Rh alloy and the Rh content of the alloy is increased considerably, sufficient resistance to spark consumption cannot be attained under certain operating conditions.
The aforementioned publication discloses endurance test results of a plug whose chip is formed from an alloy containing an Ir--Rh binary alloy as a base material and a third metal component, such as Pt or Ni, which is added to the base material in a manner of substituting for Ir. However, according to the endurance test results, the amount of consumption of a chip as observed after the endurance test is rather larger than that of a chip formed from an alloy into which neither Pt nor Ni is added, indicating that no improvement is achieved in the consumption resistance of such an Ir--Rh binary alloy.